Thermal ink jet printing has been described in many technical publications, and one such publication relevant to this invention is the Hewlett Packard Journal, Volume 36, Number 5, May 1985, incorporated herein by reference.
In the manufacture of thermal ink jet printheads, it is known to provide conductive traces of aluminum over a chosen resistive material, such as tantalum-aluminum, to provide electrical lead-in conductors for conducting current pulses to the lithographically defined heater resistors in the resistive material. These conductive traces are formed by first sputtering aluminum on the surface of a layer of resistive material and thereafter defining conductive trace patterns in the aluminum using conventional photolithographic masking and etching processes.
It is also known in this art to deposit an inert refractory material such as silicon carbide or silicon nitride over the aluminum trace material and the exposed resistive material in order to provide a barrier layer between the resistive and conductive materials and the ink. This ink is stored in individual reservoirs and heated by thermal energy passing from the individually defined resistors and through the barrier layer to the ink reservoirs atop the barrier layer. The ink is highly corrosive, so it is important that the barrier layer be chemically inert and highly impervious to the ink.
In the deposition process used to form the barrier layer for the above printhead structure, rather sharply rounded contours are produced in the barrier layer material at the edges of the conductive aluminum traces. These contours take the form of rounded edges in the silicon carbide layer which first extend laterally outward over the edges of the aluminum traces and then turn back in and down in the direction of the edge of the aluminum trace at the active resistor area. Here the silicon carbide barrier material forms an intersection with another, generally flat section of silicon carbide material which is deposited directly on the resistive material. This intersection may be seen on a scanning electron microscope (SEM) as a crack in the barrier layer material which manifests itself as a weak spot or area therein. This weak spot or area will often become a source of structural and operational failure when subjected to ink penetration and to cavitation-produced wear from the collapsing ink bubble during a thermal ink jet printing operation.
In addition to the specific problem with the above prior art approach to thin film resistor substrate fabrication, it has been found that, in general, thin films and fluidic cavities in these structures which have been optimized for superior printing speed and print quality suffer from short printing resistor operating life. This is especially true when large over-energy tolerance is required. Resistor aging curves taken throughout the printing life of a thermal ink jet heater resistor reveal strongly two mechanisms which contribute to the early demise of the heater resistor. One is rapid resistor value increase due to electrochemical and mechanical interactions near the resistor terminations. The second is a slow but continuous increase of the resistance caused by the interface oxidation with the thermal standoff layer and a passivation layer. Simply stated, any mechanism contributing to the increase of the resistor value in ohms is a mechanism that leads toward the final resistor failure when its value is infinite.